1. Field of the Invention
The present invention relates to a gradient-index optical element used as optical lenses for cameras, microscopes, endoscopes, etc., and a method of making the same.
2. Discussion of Related Art
A gradient-index optical element (hereinafter often called a GRIN lens) is a new optical element having a lens action based on a refractive index gradient or profile across a lens medium. This optical element has an unheard-of ability to make correction for aberrations. In particular, a radial type of gradient-index optical element (r-GRIN for short) having a refractive index profile in its radial direction can make correction for aberrations uncorrectable even by use of aspheric lenses, for instance, curvature of field and chromatic aberrations, and so now attracts attention as one next-generation optical element.
The GRIN lens has the feature of being able to make correction for chromatic aberrations and, hence, can be effectively applied to lens systems handled with white light sources, for instance, cameras, and microscopes. The property of the GRIN lens, by which chromatic aberrations can be corrected, is called a low or negative dispersion profile property. Roughly speaking, the refractive index of the element in its diametrical direction changes from the state of high refractive index and low dispersion to the state of low refractive index and high dispersion. Some element constructions of the GRIN lens having such property are disclosed in U.S. Pat. Nos. 5,166,827, 5,366,939 and 5,349,493. The dispersion profile property of a medium is represented by its Abbe number, say, V.sub.10 (=N.sub.d10 /(N.sub.F10 -F.sub.c10)). The Abbe number of a medium implies that the smaller the positive value thereof, the larger the chromatic aberrations produced in the medium is. Such property is called a high dispersion profile property.
When V.sub.10 has a large positive value, there is achieved a so-called low dispersion profile where the chromatic aberrations produced in the medium are reduced. When V.sub.10 is infinitely great, there is obtained a zero dispersion profile where no chromatic aberrations are produced at all.
When V.sub.10 has a negative value, there is obtained a so-called low dispersion profile. With the negative dispersion profile, too, the larger the negative value, the smaller the chromatic aberrations produced are, and the smaller the negative value, the larger the chromatic aberrations produced are. Unlike the case of a high dispersion profile-low dispersion profile where the direction of chromatic aberrations produced has a positive value, however, red or other light of longer wavelength increases in the amount of refraction while blue or other light of shorter wavelength decreases in the amount of refraction. Such chromatic aberrations are a unique phenomenon that is not found in the case of glass lenses. Thus, a lens material having such chromatic aberrations is believed to open up a new possibility for optical system designs.
Particularly effective in this regard is a gradient-index optical element having a low dispersion profile where chromatic aberrations are less produced, and a negative dispersion profile where chromatic aberrations are produced in a direction opposite to an ordinary direction.
The GRIN lens has two important properties; one is that no chromatic aberrations are produced in the medium, and another is that a refractive index difference .DELTA.n between the optical axis and the periphery of the lens--which determines the amount of bending of light through the medium--is large. The larger the .DELTA.n value, the larger the bending of light rays through the GRIN lens is, and so the lens action per unit length of the GRIN lens becomes great. It is thus possible to make the length of the lens short. This in turn can not only achieve size reductions of lens systems, but also achieve high performance by making use of large refracting power, for instance, a wide field angle available to lens systems.
.DELTA.n is determined by a concentration difference of dopant between the center and the periphery of a GRIN lens, which dopant imparts a gradient-index across the GRIN lens. To obtain a GRIN lens having a large .DELTA.n, it is thus required to give a large concentration difference of the dopant across the lens.
For glass compositions that can provide inexpensive glasses having such properties on industrially large scales and in stable manners, U.S. Pat. No. 5,448,409 discloses an SiO.sub.2 --BaO--TiO.sub.2 --K.sub.2 O composition system. Although the glass composition disclosed therein provides stable glasses and is suitable for mass production, yet it is difficult to obtain .DELTA.n values of about 0.02 or larger. In this SiO.sub.2 --BaO--TiO.sub.2 --K.sub.2 O composition system, the refractive index profile-determining dopant is a barium component, and it is difficult to impart a large concentration difference across the barium component. Thus, there is a certain limit on the magnitude of .DELTA.n achievable within the composition range disclosed therein.
The reason no large value can be obtained for .DELTA.n is that no sufficient barium concentration difference is obtained between the center and the periphery of the element. Upon a careful examination, it is noted that the reason no sufficient barium concentration difference is obtained between the center and the periphery of the element is that it is difficult to lower the barium concentration at the periphery of the element.
The impartment of a concentration profile to barium, for instance, is achieved by dipping gel in alcohol or other solvent for a certain time, thereby allowing barium present in the gel to be diffused and eluted in the dipping solution. This is presumed to render it difficult to lower the concentration of barium at the periphery of the element.
The diffusion and elution of barium take place due to a difference between the concentration of barium in the gel and the concentration of barium in the treating solution in which the gel is dipped. Since the amount of the treating solution in which the gel is dipped is limited, however, the concentration of barium in the treating solution increases with the barium component eluted out of the gel until there is no concentration difference between inside and outside the gel, putting a stop to the diffusion of barium. It is thus impossible to make the concentration of barium at the periphery of the gel lower than the concentration of barium in the dipping solution.
On the other hand, it may be possible to continuously remove the barium component eluted in the dipping solution. However, such continuous removal of the barium component from the dipping solution offers an unstable factor to the dipping treatment, and so is not practical for strictly controlling the concentration of barium in the gel with high reproducibility.
A chief object of the present invention is to provide, with stable quality and ease, a gradient-index optical element material having a low dispersion profile and a negative dispersion profile and so effective for application to optical equipment, and having a large .DELTA.n value. A particular object of the present invention is to provide a gradient-index optical element material which has a concentration profile across a barium component, and is of excellent quality.